fab library

The antigen-binding (Fab) fragment is a region on an antibody that binds to antigens. The “fab fragment” (“fragment, antigen binding”) was originally defined as one of the cleavage products after treatment of rabbit IgG with papain, which cleaves the core hinge, resulting in two identical fab fragments and the intact Fc as products. It contains four domains: the heavy chain variable domain (VH) linked to constant domain 1 (CH1), and the light chain variable domain (VL) linked to a constant domain (CL). Fab libraries, in which light-chain (LC, contains κ chain or λ chain) genes and heavy-chain (HC, here we only illustrate γ chain) variable region genes as well as CH1 genes are cloned into a phagemid vector and subsequently displayed on the surface of the filamentous phage particle, have been widely used for the isolation of antibodies with specificity for haptens, foreign antigens, and self antigens. The isolation of Fabs from combinatorial libraries is thus valuable in contributing to the understanding of Antibody–Antigen interactions, as well as the nature of the in vivo immune response.

Compared with scFv libraries, the construction of Fab libraries has the advantage of simplicity in technically, because scfv fragments have more tendencies to form dimers and higher order multimers in a clone-dependent and relatively unpredictable way. Multimeric antibody molecules bind more strongly to immobilized antigen than monomeric fragments because of their greater avidity, and therefore have higher “apparent” affinities. This explains why an accurate determination of the affinity is not easily possible with mixtures of mono- and multimeric scFv fragments.

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